Ever since he was charged with perverting the course of justice in February 2012 after allegedly asking his former wife Vicky Pryce to accept speeding points on his behalf, Chris Huhne has insisted he is an innocent man. But today as his trial opened at Southwark crown court, the former Lib Dem Energy Secretary stunned everyone by changing his plea to guilty.

While Huhne could technically remain as an MP if imprisoned for less than a year, the Lib Dems will now almost certainly face a difficult by-election in Eastleigh, where they currently have a majority of 3,864 and where the Conservatives finished second in 2010.

For the Tories, who have included 20 Lib Dem MPs on their 2015 target list of 40, the contest will be an early test of their ability to take seats off Clegg's party. But with UKIP likely to pour resources into the constituency (Nigel Farage will surely consider standing), a split in the right-wing vote could yet save the Lib Dems. The contest is also a test of whether Labour supporters are still prepared to vote tactically for the Lib Dems in order to keep the Tories out.

To have any hope of achieving a majority in 2015, the Conservatives have to win seats like Eastleigh. If they fail to do so, and if Cameron's EU referendum pledge proves to have done little to dent UKIP's appeal, Tory MPs will feel emboldened to voice further doubts over the PM's leadership.

Update: In a short statement outside the court, Huhne said: "I have pleaded guilty today. I am unable to say more while there is an outstanding trial. But having taken responsibility for something that happened 10 years ago the only proper course of action for me is to resign my Eastleigh seat in parliament which I will do very shortly. And that's all I'm able to say today."

The by-election is on. Nigel Farage has said he will decide in the next 24-48 hours whether to stand.